1. Field of the Invention
This invention relates generally to multilayer ceramic capacitors, and more particularly to a long life, high energy, multilayer ceramic capacitor having a high dielectric constant and of a reduced size to be highly suitable for implantation in the human body.
2. Discussion of the Related Art
Multilayer ceramic capacitors have been available having a variety of electrical characteristics. Barium titanate (BaTiO.sub.3) has commonly been used for ceramic multi-layer capacitors. This base body material, or possibly several alternative dielectric materials, coupled with specifically identified additives, are discussed in Burn, Ceramic Capacitor Dielectrics, in Engineered Materials Handbook, Vol. 4, Ceramics and Glasses, ASM International (1991). These capacitors have generally had certain limitations which made them unsuitable for implantation in the human body, for example, as part of a cardiac defibrillator. Heart pacers and defibillators are included within the term "implantable medical devices." In general, ceramic capacitors lacked the energy density required to support the primary discharge for defibrillators.
For implantable medical devices, which typically require short duration high energy pulses, a major portion of the power supply for these devices is a capacitor. A defibrillator, for example, uses a capacitor to be charged and store energy from the battery, and then release this energy in the form of an electrical therapy pulse to the heart. The primary discharge source for defibrillators has been relatively large electrolytic capacitors. These have been used with implantable medical devices because they are able to achieve at least some of the major parameters necessary for such an environment. However, these capacitors have significant limitations for defibrillator use. Their size has been a major drawback. Such implantable electrolytic capacitors would typically be about 0.63 inches in diameter and about 1.9 inches long and would weight about 14.7 grams. Two such capacitors would be required to achieve the energy level, 30 J at 700 volts, required for defibrillators. The two electrolytic capacitors typically have a volume of about 28 cc. Because of their cylindrical shape, another 10 cc is wasted so their effective volume is about 38 cc. The reason such a capacitor must be of this relatively large size is that electrolytics typically have an energy density of 1-2 J/cc, so making it large is the only way to obtain the required 30 J energy level. Because of its size, a defibrillator with such electrolytic capacitors could only be implanted in the abdomen of a person, requiring electrical wires to extend from that location to the electrodes at the heart.
Electrolytic capacitors have other potential disadvantages. They tend to outgas, so provisions must be made in the implanted device to contain and neutralize such gases. Further, electrolytics need to be reformed on a periodic basis. This requires that a voltage be applied and held for a predetermined period of time, thereby reducing the useful life of the battery in the implanted device. Their useful life is about five years, so periodic replacement is necessary. The leakage current of implantable electrolytic capacitors is typically 0.1-1.0 ma and their dissipation factor is as high as about 10%.
Because of various considerations, it is desirable that the electrical wire extending from the implanted device to the cardiac muscle be as short as possible. Also, the operation to implant a device in the upper chest is much more minor than is the abdominal operation necessary to implant a large device. Due to the relatively large size of the capacitors as a portion of the overall implanted device, it has not previously been possible to implant the device in relatively near proximity to the heart.
Characteristic limitations in multilayer ceramic capacitors in the past have made them not suitable for the primary high capacitance, high voltage discharge required for defibrillators. Among these limitations are relatively low dielectric constant, in the range of 2,000 to 4,000 at operational temperature, leakage current of several hundred microamps, low capacitance, typically less than 20 .mu.f, and relatively low breakdown voltage of not greater than 30 volts. They were not able to deliver 30 J of energy at 700 volts as is required for defibrillators. These limitations prevented the necessary high peak voltage of short duration required for such devices for implanted human medical use.
Another significant limitation of ceramic capacitors is the ferroelectric effect where capacitance has in the past fallen off very sharply with increased DC bias voltage. For example, where capacitance might be in the range of 4.75 .mu.f at 20 volts, it might fall to 1.35 .mu.f at 200 volts and 0.7 .mu.f at 340 volts. By way of contrast, for implantable devices a capacitor must operate at least at 700 volts without significant reduction in its capacitance. Thus, because of this striking ferroelectric effect, ceramic capacitors have not previously been thought to be applicable to implantable medical devices requiring a relatively high energy output.